1. Field of the Invention
The present invention relates to a developing apparatus and particularly to a developing apparatus provided with a function to separate and remove different colored toner mixed into a developer.
2. Description of the Prior Art
A two color electrophotographic recording apparatus has been proposed in which the surface of a photoconductive photosensitive recording medium is uniformly charged, then the first exposure is performed to discharge the region to which a first toner is to be attached to thereby form a first electrostatic latent image, then the first electrostatic latent image is subject to reversal development by the first developing apparatus to thereby form a first toner image, then the uniformly formed charge residual region is subject to the second exposure to form a second electrostatic latent image, and then the second electrostatic latent image is subject to reversal development to thereby form the second toner image. When the photoconductive photosensitive recording medium is the final one, the two toner images are fixed as they are on the recording medium. In the case where the photoconductive photosensitive recording medium is an intermediate one such as photosensitive drum which is repeatedly used, the two toner images are transferred onto a sheet of recording paper such as a sheet of plain paper and then fixed thereon.
In such a recording apparatus, the development of the second electrostatic latent image by using the second developing apparatus is performed to the recording medium surface at which the first toner is held by electrostatic force and therefore when the second developer of the second developing apparatus is applied onto the recording medium surface, a part of the toner constituting the first toner image is removed therefrom and mixed into the second developer. If the operation time of the recording apparatus is prolonged, the mixed toner is accumulated in the second developing apparatus. As the amount of the first toner mixed in the second developer increases, the second toner image becomes muddy so that clear two-color image can not be obtained. In order to prevent the turbidity of the second toner image, a developing apparatus has been proposed which is provided with a mixed toner removing means for removing the first toner mixed into the second developer. The mixed toner removing means is arranged such that by utilizing the difference in polarity of charge between the first toner and the second toner in the second developer, an electrostatic attraction roll applied with a voltage of the same polarity as the second toner (that is the different polarity from the first toner) is employed to attract the first toner mixed in the second developer to thereby remove the first toner from the second developer. In the case where the first toner in the first developing apparatus is charged with the same polarity as the second toner in the second developing apparatus, it is arranged such that the charge polarity of the first toner is reversed when it is mixed into the second developer.
Since the developer moves in the developing apparatus in the form of layer flow, it is impossible to cause the electrostatic attraction force to effectively act on the first toner mixed into the inside of the layer of the second developer and therefore the mixed toner can not be sufficiently removed.